Microfluidic devices are well known and are used for a variety of purposes including applications such as lab-on-a chip systems. A microfluidic chip includes at least one channel having at least one dimension of the order of micrometers or tens of micrometers. These channels are typically formed in a substrate such as glass, silicon or a polymer such as PDMS. Individual channels that form the microfluidic chip are in fluid communication with one another and may be connected to achieve a desired effect such as a mixing, pumping, redirection or allowing specific chemical reactions to occur within the chip.
It is known to fabricate such microfluidic chips on a rotatable substrate so as to use an induced centrifugal field resultant from a rotation of the substrate to bias fluid movement within the channels of the chip. Such a force acts outwardly from an axis of rotation of the substrate such that during rotation of the substrate a fluid within a microfluidic channel will tend to move out and away from the axis of rotation of the substrate towards a circumference of the substrate. It will be appreciated that the actual force is in a direction parallel to radial lines extending outwardly from the axis of rotation but the fluid motion will be constrained by the actual pattern of the channels within which it flows.
In an earlier patent application of the instant applicant—US2012040843A1—there is described a microfluidic device that uses an array of single-cell capture traps which resemble cups in the geometry of a letter “V”. These capturing structures are placed in the sedimentation path of a cellular sample when processed using centrifugal forces to migrate a cell population microfluidic chip in a stopped-flow where the suspending liquid is essentially at rest with respect to the rotating substrate. Using such an arrangement it is possible to isolate and enumerate a number of candidate particles or groups of particles. Example of such particles are circulating tumour cells (CTCs). A CTC is a cell that has detached from a solid tumour and entered into the peripheral bloodstream. Recent progress in the field of oncological diagnostics, however, has also identified the presence of multi-cellular clusters of CTCs in the blood as a key diagnostic and prognostic of patient fate, particularly in terms of development of resistance to chemotherapy. These cells are present in a sample of blood and using a device per the teaching of US2012040843A1 it is possible to identify these cells at single-cell resolution, following sample preparation using a negative-isolation mode of CTC enrichment. However, due to the single-cell resolution, the V-cup strategy is not suitable for isolation of multi-cellular events but the fact remains that it is desirable to be able to accurately detect such CTCs.
Current commercial devices for the detection of CTCs are based on “positive-mode” isolation where cells are directly targeted based on assumed phenotypes. These arrangements require a physical or molecular bias, such that the techniques require knowledge of the phenotype of candidate cells within a sample in order to capture the cells of interest. It would be useful if CTCs or other target molecules could be isolated without such bias.
Other techniques aim to isolate candidate cells via assumed bio-physical properties of candidate CTCs and are typically based on the presumed increased size of such cells. Such systems mediate size filtration by a porous filter or a track-etched membrane. These membranes are designed and directed at enriching CTCs that exist as single cells, and cannot distinguish between a potential CTC cell cluster and a collection of single cells that strike and resolve to the membrane at the same point but exist as single cells in the blood.
Hence, there continues to be a need for devices and methodologies that would facilitate the targeted selection of molecules or groups of molecules based on their physical dimensions.